High-excitation molecular gas in local luminous AGN hosts

TL;DR

This study observes highly excited molecular gas in local luminous AGN hosts using JCMT, revealing high CO line ratios and suggesting shocks or star formation as excitation mechanisms, with implications for future high-resolution ALMA studies.

Contribution

It provides new observations of high-excitation molecular gas in local AGN hosts, highlighting the role of shocks and star formation, and discusses the potential of ALMA for detailed future studies.

Findings

High CO (3--2)/(1--0) ratios indicate highly excited molecular gas.

An exceptionally bright CO J=4--3 line in 3C 293 suggests shock excitation.

Star formation and AGN activity both contribute to gas excitation.

Abstract

We used the mm/sub-mm receivers on the James Clerk Maxwell Telescope (JCMT) to observe the CO J=3--2, 2--1 lines in five local, optically powerful AGN and the J=4--3 line in 3C 293 (a powerful radio galaxy). Luminous CO J=3--2 emission and high CO (3--2)/(1--0) intensity ratios are found in all objects, indicating highly excited molecular gas. In 3C 293 an exceptionally bright CO J=4--3 line is found which cannot be easily explained given its quiescent star-forming environment and low AGN X-ray luminosity. In this object shocks emanating from a well-known interaction of a powerful jet with a dense ISM may be responsible for the high excitation of its molecular gas on galaxy-wide scales. Star formation can readily account for the gas excitation in the rest of the objects, although high X-ray AGN luminosities can also contribute significantly in two cases. Measuring and eventually imaging CO line ratios in local luminous QSO hosts can be done by a partially completed ALMA during its early phases of commissioning, promising a sensitive probe of starburst versus AGN activity in obscured environments at high linear resolutions.